This invention relates in general to electronic controllers for electric trailer brakes and in particular to a load short circuit protection device having an audible alarm.
Towed vehicles, such as recreational and utility trailers adapted to be towed by automobiles and small trucks, are commonly provided with electric brakes. The electric brakes generally include a pair of brake shoes which, when actuated, frictionally engage a brake drum. An electromagnet is mounted on one end of a lever to actuate the brake shoes. When an electric current is applied to the electromagnet, the electromagnet is drawn against the rotating brake drum and pivots the lever to actuate the brakes. Typically, the braking force produced by the brake shoes is proportional to the electric current applied to the electromagnet. This electric current can be relatively large. For example, the brakes on a two wheeled vehicle can draw six amperes of current when actuated and the brakes on a four wheeled vehicle can draw 12 amperes of current.
The towed vehicle brakes are typically activated by an electric brake controller mounted within the towing vehicle. The simplest electric brake controller includes a switch connected between the towing vehicle power supply and the towed vehicle electric brakes. Usually a resistor is included between the switch and the brakes to limit the current supplied to the brakes. The towing vehicle operator closes the switch to activate the towed vehicle brakes. Such brake controllers can further include a plurality of switches. A current limiting resistance is associated with each switch to provide different braking intensities.
An electric brake controller also can include a variable resistor, such as a rheostat, which is connected between the towing vehicle power supply and the brake electromagnets on the towed vehicle. Such electric brake controllers are commonly referred to as electric brake actuators. The towing vehicle operator manually adjusts the variable resistor setting to vary the amount of current supplied to the brake electromagnets and thereby control the amount of braking force developed by the towed vehicle brakes.
Also know in the art are more sophisticated electric brake controllers which include an electronic circuit to automatically supply current to the brake electromagnets when the towing vehicle brakes are applied. Such electric brake controllers are commonly referred to as electronic brake controllers and typically include a sensing unit which generates a brake control signal corresponding to the desired braking effort. For example, the sensing unit can include a pendulum which is displaced from a rest position when the towing vehicle decelerates and an electronic circuit for generating a brake control signal which is proportional to the pendulum displacement. One such sensing unit is disclosed in U.S. Pat. No. 4,721,344.
Known electronic brake controllers also typically include an analog pulse width modulator. The input of the pulse width modulator is electrically connected to the sensing unit and receives the brake control signal therefrom. The pulse width modulator is responsive to the brake control signal for generating an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in proportion to the magnitude of the brake control signal. Thus, the duty cycle of the pulse train corresponds to the amount of the braking effort desired.
The output of the pulse width modulator is usually coupled to an output stage which includes one or more power transistors connected between the towing vehicle power supply and the towed vehicle brake electromagnets. The power transistors act as electronic switches, being cycled between conducting and non-conducting states in accordance with the duty cycle of the output signal. One such electronic brake controller is disclosed in U.S. Pat. No. 5,620,236.